A belt retractor with the features mentioned above has been described in document DE 10 2009 024 292 AI. Since the two-stage force limiting device consists of two torsion bars arranged in the interior of the belt shaft of the belt retractor and are coupled to one another by means of a connecting tube, each having a different force limiting level, the connecting tube is coupled to the belt shaft via a separate coupling element. If the associated profile head is locked to the housing frame accommodating the belt shaft, the force limiting device allows a further force-limited rotation of the belt shaft in the belt extension direction with respect to the locked profile head during an increase in the belt force caused by a forward displacement of the belted passenger after a preset force limiting level has been exceeded. In order to configure the two-stage force limitation, a first torsion bar having a high force limiting level is firmly connected by one end to the profile head, while its other end is connected to the connecting tube enclosing it; the second torsion bar, arranged in a line with the first torsion bar, and having a lower force limitation level, is connected by its one end to the connecting tube and by its other end to the belt shaft. Since at the beginning of the force limitation, only the low force limitation level should be enabled, the connecting tube connecting the two torsion bars to one another is not yet connected to the belt shaft, so that the other end of the second torsion bar having the lower force limitation level connected to the belt shaft on one side is connected to the profile head firmly locked to the housing via the connecting tube and via the first torsion bar having the high force limitation level. If the belt shaft now continues rotating as a result of the forward displacement when the profile head is locked, only the second torsion bar with the lower force limitation level is consequently twisted. If the higher force limitation level is to be actuated at a preset angle of rotation of the shaft with respect to the fixed profile head, the connecting tube is coupled to the rotation of the belt shaft via the correspondingly provided coupling element. Owing to the coupling of the connecting tube to the belt shaft, both ends of the second torsion bar having the lower force limitation level, are now connected to the belt shaft, so that if the second torsion bar is bridged, the first torsion bar acted upon by the rotation of the belt shaft via the coupling element and the connecting tube, is now twisted with respect to the fixed profile head.
The coupling element configured between the connecting tube and the belt shaft enclosing the latter is formed by a spring element arranged in a recess of the belt shaft; at the beginning of the rotation of the belt shaft, said spring element slides over the surface of the connecting tube with a locking hook configured thereon. The rotational path of the belt shaft provided for the actuation of the high force limiting level is fixed by the arrangement of a locking groove extending in the longitudinal direction of the connecting tube and arranged therein, so that after the passage of the thus preset rotational path of the belt shaft with the coupling element attached thereto, the locking hook of the coupling element drops into the locking groove of the connecting tube, and the belt shaft and the connecting tube are connected to one another by means of the coupling element, which is supported on one side in the recess of the belt shaft, and on the other side to the locking hook in the locking groove of the connecting tube.
The disadvantage of the known belt retractor is that switching the force limiting element from the lower force limiting level to the higher force limiting level has to already take place before reaching a full rotation of the belt shaft because of the separation between the initial position of the locking hook of the coupling element and the position of the locking groove in the connecting tube, which is only possible at the beginning of the force limitation. On the other hand, however, there is also a requirement that performing such switching even later, namely, after more than one rotation of the belt shaft with respect to the locked profile head, is possible.
It is therefore the underlying object of the invention to configure a belt retractor having the features mentioned above, such that switching from the low to the high force limiting level can also be arranged at a later time than a full rotation of the belt shaft with respect to the locked profile head.